1. Field of the Invention
The present invention relates to an automobile antenna system, and more particularly, to an improved automobile antenna system which prevents deflection of the output level of a main antenna provided on the rear portion of an automobile.
2. Description of the Prior Art
Generally, it is difficult to receive stable broadcast waves by means of an antenna during the travel of a vehicle. Especially, high-frequency waves in a VHF band such as FM broadcast waves have a strong tendency to experience rectilinear propagation, so that direct waves and other waves involving reflection from buildings, mountains or other obstacles interfere with each other and cause multipath phenomenon such as distortion or momentary interruption of sound.
To solve this problem, a diversity-reception type antenna system has been proposed, in which two antennas are disposed on the vehicle body at predetermined intervals so that the receiving operation is taken over by the antenna which enjoys superior reception by automatic change-over, whereby the directivity of the antenna is improved and the amount of multipath noise is lowered.
Some diversity-reception type automobile antenna systems are composed of two glass antennas provided on the upper portion and the lower portion of the rear windshield, which serves as the main antenna and the sub antenna, respectively, of a diversity-reception antenna.
However, broadcast electric waves 12 which are transmitted from a broadcasting station 10 produce stationary waves 16 at certain places when reflected by a building or the like, as shown in FIG. 9. If a vehicle which is receiving the electric waves 12 drives into the area where the stationary waves 16 are being produced and the main antenna of the vehicle is situated at the node of the stationary wave 16, the output of the main antenna suddenly drops and the receiving quality is deteriorated. It is sometimes impossible at such times to appropriately change the main antenna over to the sub antenna for improving the receiving state under a conventional diversity-reception system which employs two antennas provided on the rear windshield.